Relevance of π-Backbonding for the Reactivity of Electrophilic Anions [B12X11]− (X=F, Cl, Br, I, CN)

Electrophilic anions of type [B12X11]− posses a vacant positive boron binding site within the anion. In a comparatitve experimental and theoretical study, the reactivity of [B12X11]− with X=F, Cl, Br, I, CN is characterized towards different nucleophiles: (i) noble gases (NGs) as σ-donors and (ii) CO/N2 as σ-donor-π-acceptors. Temperature-dependent formation of [B12X11NG]− indicates the enthalpy order (X=CN)>(X=Cl)≈(X=Br)>(X=I)≈(X=F) almost independent of the NG in good agreement with calculated trends. The observed order is explained by an interplay of the electron deficiency of the vacant boron site in [B12X11]− and steric effects. The binding of CO and N2 to [B12X11]− is significantly stronger. The B3LYP 0 K attachment enthapies follow the order (X=F)>(X=CN)>(X=Cl)>(X=Br)>(X=I), in contrast to the NG series. The bonding motifs of [B12X11CO]− and [B12X11N2]− were characterized using cryogenic ion trap vibrational spectroscopy by focusing on the CO and N2 stretching frequencies (Formula presented.) and (Formula presented.), respectively. Observed shifts of (Formula presented.) and (Formula presented.) are explained by an interplay between electrostatic effects (blue shift), due to the positive partial charge, and by π-backdonation (red shift). Energy decomposition analysis and analysis of natural orbitals for chemical valence support all conclusions based on the experimental results. This establishes a rational understanding of [B12X11]− reactivety dependent on the substituent X and provides first systematic data on π-backdonation from delocalized σ-electron systems of closo-borate anions. © 2021 The Authors. Chemistry - A European Journal published by Wiley-VCH Gmb

First steps towards a stable neon compound: observation and bonding analysis of [B 12 (CN) 11 Ne] −

Noble gas (Ng) containing molecular anions are much scarcer than Ng containing cations. No neon containing anion has been reported so far. Here, the experimental observation of the molecular anion [B12(CN)11Ne]− and a theoretical analysis of the boron–neon bond is reported

Na+[Me3NB12Cl11]−·SO2: a rare example of a sodium–SO2 complex

In the title compound, Na+[Me3NB12Cl11]−·SO2 [systematic name: sodium 1-(trimethylammonio)undecachloro-closo-dodecaborate sulfur dioxide], the SO2 molecule is η1-O-coordinated to the Na+ cation. Surprisingly, the SO2 molecule is more weakly bound to sodium than is found in other sodium–SO2 complexes and the SO2 molecule is essentially undistorted compared to the structure of free SO2. The Na+ cation has a coordination number of eight in a distorted twofold-capped trigonal prism and makes contacts to three individual boron cluster anions, resulting in an overall three-dimensional network. Although the number of known η1-O-coordinated SO2 complexes is growing, sodium-SO2 complexes are still rare

A second polymorph of bis(triphenyl-λ5-phosphanylidene)ammonium chloride–boric acid adduct

The title crystal structure is a new triclinic polymorph of [(Ph3P)2N]Cl·(B(OH)3) or C36H30NP2+·Cl−·BH3O3. The crystal structure of the orthorhombic polymorph was reported by [Andrews et al. (1983). Acta Cryst. C39, 880–882]. In the crystal, the [(Ph3P)2N]+ cations have no significant contacts to the chloride ions nor to the boric acid molecules. This is indicated by the P—N—P angle of 137.28 (8)°, which is in the expected range for a free [(Ph3P)2N]+ cation. The boric acid molecules form inversion dimers via pairs of O—H...O hydrogen bonds, and each boric acid molecule forms two additional O—H...Cl hydrogen bonds to one chloride anion. These entities fill channels, created by the [(Ph3P)2N]+ cations, along the c-axis direction

Reaction of the Tricyanoborate Dianion [B(CN)₃]^2– with HgCl₂

The very reactive [B­(CN)₃]^2– dianion has a strongly nucleophilic boron atom and can be used for the synthesis of tricyanoborates, which otherwise are difficult to access. Herein the reaction of this anion with HgCl₂ is reported. The main product is the anionic mercury complex [Hg­(B­(CN)₃)₂]^2–. Heteronuclear NMR spectroscopic experiments shows that the reaction proceeds via the intermediate [ClHgB­(CN)₃]^2–. Even though [Hg­(B­(CN)₃)₂]^2– is the main product, it is difficult to obtain it in pure form, because it slowly decomposes in the presence of water and air to [(NC)­HgB­(CN)₃]⁻. All three anions were fully characterized by hetereonuclear NMR spectroscopy (¹¹B, ¹³C, and ¹⁹⁹Hg). Single-crystal X-ray diffraction studies of the salts K­[ClHg­B­(CN)₃], [Ph₄P]₂[Hg­(B­(CN)₃)₂], K­[(NC)­Hg­B­(CN)₃], and [Ph₄P]­[(NC)­Hg­B­(CN)₃] revealed linear coordination environments around mercury for all anions. The Hg–B bonds range from 2.219(5) Å in [Hg­(B­(CN)₃)₂]^2– to 2.148(11) Å in [ClHgB­(CN)₃]⁻, are in accord with the sum of the covalent radii of mercury and boron, and can be described as covalent single bonds. A comparison with related complexes indicates that the [B­(CN)₃]^2– dianion is a stronger ligand than chloride, cyanide, or carbenes. [Hg­(B­(CN)₃)₂]^2– hydrolyses in solution only in the presence of oxygen. It is suggested that <i>cis</i>-[Hg­(OH)₂­(B­(CN)₃)₂]^2– is formed as a very unstable intermediate, which decomposes very fast to [(NC)­Hg­B­(CN)₃]⁻ and other products. The anion <i>cis</i>-[Hg­(OH)₂­(B­(CN)₃)₂]^2– would contain mercury in the unusual oxidation state +IV. Quantum-chemical calculations were performed to support this assumption

Improving the Solubility of Halogenated 1‑Ammonio-<i>closo</i>-dodecaborate Anions

The partly halogenated and <i>N</i>-alkylated <i>closo</i>-dodecaborates [B₁₂Cl₆H₅N­(propyl)₃]⁻ and [B₁₂Br₆H₅NR₃]⁻ (R = ethyl–pentyl) were prepared by alkylation of [B₁₂H₁₁NH₃]⁻ and subsequent halogenation with elemental chlorine or <i>N</i>-bromosuccinimide. Simple metathesis reactions yielded the [HNMe₃]⁺, [C₆mim]⁺, [NBu₄]⁺, and Na⁺ salts, which were characterized by heteronuclear NMR and IR spectroscopy as well as electrospray ionization mass spectrometry. The crystal structures of the salts [HNMe₃]­[B₁₂Br₆H₅N­(ethyl)₃]·CH₃CN, [HNMe₃]­[B₁₂Br₆H₅N­(propyl)₃], Na­[B₁₂Br₆H₅N­(butyl)₃], and [HNMe₃]­[B₁₂Cl₇H₄N­(propyl)₃]·CH₃CN were determined by single-crystal X-ray diffraction. The [C₆mim]⁺ salts are thermally stable to temperatures higher than 300 °C. The melting points are between 57 and 80 °C, which classify the [C₆mim]⁺ salts of [B₁₂Cl₆H₅N­(propyl)₃]⁻ and [B₁₂Br₆H₅NR₃]⁻ (R = propyl–pentyl) as ionic liquids. The anions are oxidized only at potentials higher than 2 V versus Fc^0/+ as determined by cyclic voltammetry. The solubility of the sodium salts in CH₂Cl₂ solution was determined by NMR spectroscopy. With the increasing length of the alkyl chain attached to the ammonio group the solubility is significantly enhanced. A solubility up to 125 mmol/L for Na­[B₁₂Br₆H₅N­(pentyl)₃] in dichloromethane was determined. In addition, the trialkylation of the perchlorinated anion [B₁₂Cl₁₁NH₃]⁻ was investigated in detail. A Hofmann elimination was observed to occur at higher temperatures, when alkyl groups with β-hydrogen atoms were introduced. Organic substituents without β-hydrogen atoms gave more stable compounds; however, trialkylation proved to be difficult presumably due to steric hindrance. The crystal structure of the byproduct [PPh₄]₂[B₁₂Cl₁₁N­(propargyl)₂] was determined

Properties of gaseous closo-[B6X6]2− dianions (X = Cl, Br, I)

Electronic structure, collision-induced dissociation (CID) and bond properties of closo-[B6X6]2− (X = Cl–I) are investigated in direct comparison with their closo-[B12X12]2− analogues. Photoelectron spectroscopy (PES) and theoretical investigations reveal that [B6X6]2− dianions are electronically significantly less stable than the corresponding [B12X12]2− species. Although [B6Cl6]2− is slightly electronically unstable, [B6Br6]2− and [B6I6]2− are intrinsically stable dianions. Consistent with the trend in the electron detachment energy, loss of an electron (e− loss) is observed in CID of [B6X6]2− (X = Cl, Br) but not for [B6I6]2−. Halogenide loss (X− loss) is common for [B6X6]2− (X = Br, I) and [B12X12]2− (X = Cl, Br, I). Meanwhile, X˙ loss is only observed for [B12X12]2− (X = Br, I) species. The calculated reaction enthalpies of the three competing dissociation pathways (e−, X− and X˙ loss) indicated a strong influence of kinetic factors on the observed fragmentation patterns. The repulsive Coulomb barrier (RCB) determines the transition state for the e− and X− losses. A significantly lower RCB for X− loss than for e− loss was found in both experimental and theoretical investigations and can be rationalized by the recently introduced concept of electrophilic anions. The positive reaction enthalpies for X− losses are significantly lower for [B6X6]2− than for [B12X12]2−, while enthalpies for X˙ losses are higher. These observations are consistent with a difference in bond character of the B–X bonds in [B6X6]2− and [B12X12]2−. A complementary bonding analysis using QTAIM, NPA and ELI-D based methods suggests that B–X bonds in [B12X12]2− have a stronger covalent character than in [B6X6]2−, in which X has a stronger halide character

Synthesis, Electronic Properties and Reactivity of [B12X11(NO2)]2− (X=F–I) Dianions

Nitro-functionalized undecahalogenated closo-dodecaborates [B12X11(NO2)]2− were synthesized in high purities and characterized by NMR, IR, and Raman spectroscopy, single crystal X-diffraction, mass spectrometry, and gas-phase ion vibrational spectroscopy. The NO2 substituent leads to an enhanced electronic and electrochemical stability compared to the parent perhalogenated [B12X12]2− (X=F–I) dianions evidenced by photoelectron spectroscopy, cyclic voltammetry, and quantum-chemical calculations. The stabilizing effect decreases from X=F to X=I. Thermogravimetric measurements of the salts indicate the loss of the nitric oxide radical (NO.). The homolytic NO. elimination from the dianion under very soft collisional excitation in gas-phase ion experiments results in the formation of the radical [B12X11O]2−.. Theoretical investigations suggest that the loss of NO. proceeds via the rearrangement product [B12X11(ONO)]2−. The O-bonded nitrosooxy structure is thermodynamically more stable than the N-bonded nitro structure and its formation by radical recombination of [B12X11O]2−. and NO. is demonstrated. © 2020 The Authors. Published by Wiley-VCH Gmb